Wednesday, November 22, 2017

Why Write an Assembler?

With the disassembler coming together so easy and a nice table of op codes and their associated mnemonics, it seemed to me that I had everything to write an assembler as well. This was not part of my original plans for an emulator but the idea of having an emulator that was able to take source code and let you edit it while running the program would be nice. When I implement the interpreter I am going to need to write a lot of small chunks of assembly language and step over it to make sure things are working. A built-in assembler would greatly aid this work and would even be very handy for game development.

Looking at the structure for my table, I realized that looking up mnemonics and finding the associated OP Codes would be trivial to implement. A map of mnemonics can be created with each mnemonic having a list of the different variants of the instructions and the associated OP code. If you know the mnemonic and the address mode, the OP code would simply be a traversal of the list. Here is the code for generating the map.

for (inst in m6502.commands) {
    if (mapOfOpCodes.containsKey(inst.OPString))
    else {
        mapOfOpCodes.put(inst.OPString, arrayListOf<M6502Instruction>(inst))

Traditional 6502 assembly language is not that difficult to parse. Or at least it does not appear to be. It should be possible to write a simple tokenizer and a simple parser to break down the assembly language and determine which address mode it is. Of course, there are other aspects to assembling such as directives and labels but how hard can they be? As it turns out a bit harder than I expected but not really that hard.

While the assembler was not in my original design goals, with the above code coming into my head while I was still getting the disassembler written, I managed to convince myself that this would be an interesting path to explore. As I write this I have the assembler functioning and a partially implemented solution for labels so it is well on it’s way. Development on it is now slow as I am using most of my spare time to finish my Santa trilogy. The first of the three games will be posted this Friday since even in Canada Black Friday is when the Christmas season begins. Or at least the shopping part.

Next week will be a look at the game which was developed without Adobe Animate CC, though still using the Create.js libraries. If you don’t have money to spend on Creative Cloud but have Action Script or Flash experience then this is actually a viable path. Even without Flash experience, the Create.js library is a good library to use for JavaScript games. More on that next week.

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